Long-distance vocal communication in a widely dispersed, semi-solitary species often mediates individuals’ ranging behavior and social relationships. Long calls are the most conspicuous long-distance vocalization in orangutans. They are emitted only by flanged males. Long calls are audible up to 1 kilometer for human observers on the ground and are therefore key regulators of encounters between dispersed individuals in a dense rainforest where visibility is limited. Two functions of long calls are generally considered: repulsion of male rivals and mate attraction. Nevertheless, long call function is poorly understood, given that recent work in our group has shown that: (i) there is a third likely function, travel coordination with non-receptive females; (ii) long calls can be divided into acoustically distinct types; and (iii) social systems, and in particular male-male competition and male-female relationships, differ between Sumatran and Bornean orangutans. In this project, we intend to examine the possible functions of long calls and their information content on both islands, in order to elucidate the flexibility of vocal signaling in a geographically variable social system.